Polarizing devices are of utilized in numerous optical systems and are commonly used in various applications such as liquid-crystal displays and optical isolators. The present invention relates to grid polarizers also referred to as wire-grid polarizers. Grid polarizers are well known in the art and in their simplest form, generally include a glass substrate on top of which metallic stripes (or wires) are deposited. As illustrated in FIG. 1, the separation between grid lines is such that only the zero order propagates for the wavelength range of interest. Typical metals utilized include Aluminum (Al), Silver (Ag), Gold (Au), Copper (Cu), and Chrome (Cr). In general, grid polarizers operate as follows. Contrary to the substrate where most electrons are bound to atoms or molecules, in the metal there is a high density of free electrons that oscillate in response to an external electromagnetic field. If the external field is polarized along the orientation of the grid (TE polarization), it excites the electrons causing re-emission of the incident radiation. If the external electromagnetic field is oriented perpendicular to the grid (TM polarization) a reduced number of electrons oscillate resulting in most of the incident illumination being transmitted. The result is that the basic properties of a grid polarizer include the following: (1) high transmission of TM polarization (TTM); (2) low reflection of TM polarization (RTM); (3) high reflection of TE polarization (RTE); and, (4) low transmission of TE polarization (TTE).
In order to characterize the performance of grid polarizers it is common to use two parameters: insertion loss and contrast ratio. The insertion loss, directly related to the total transmission of the polarizer, is defined asIL=−10 log10TTM,  (1)while the contrast ratio is defined as
                              CR          =                      10            ⁢                                                  ⁢                          log              10                        ⁢                                          T                TM                                            T                TE                                                    ,                            (        2        )            with both quantities being measured in dB. It is generally desired to minimize the insertion loss and maximize contrast ratio. As can be seen from definition (1), if all TM polarization is transmitted TTM=1 then the resultant IL=0 dB. For high contrast ratio the transmitted illumination must be essentially of a single polarization (TM), implying that TTE should tend to zero. In this case the contrast ratio tends to infinity. In practice, however, typical grid polarizers exhibit values of contrast ratio which are in the range 20–60 dB and insertion losses typically below 0.5 dB.
One variation to the basic grid polarizer geometry illustrated in FIG. 1 is found described in U.S. Pat. No. 5,748,368, wherein it discloses a structure comprising of a lamination of the grid between two substrates so that the metal grid is embedded in a medium exhibiting a higher index than air. As a result of this approach the insertion loss can be considerably reduced over a limited spectral region.
Another variation is disclosed in U.S. Pat. No. 6,122,103, wherein it discloses a grid polarizer that incorporates homogeneous thin-film layers between the substrate and the metal grid or further includes etches incorporated into the substrate. The goal of this modified polarizer is to extend the wavelength range of operation, particularly in the visible between 400 nm and 700 nm. Both of these aforementioned approaches to grid polarizers focus on improving performance as measured by insertion loss and contrast ratio but without significantly affecting the reflective properties of the polarizer.
There are, however, applications where it is desired that the polarizer exhibit low insertion loss, high contrast ratio, and in addition a substantial suppression of the reflected TE polarization component. An example of such a device is the optical isolator, which allows essentially one-way propagation of radiation. The typical optical isolator includes two polarizers and a Faraday rotator. The polarizers have their axis of polarization rotated by 45°. To meet increasing requirements in component stability and compactness it is desirable to integrate the various components of the isolator by having grid polarizers patterned directly into the Faraday rotator material. The integrated optical isolator incorporates the polarizing and polarization-rotation function within a single device. One difficulty with this apparatus is that back-reflected light coming towards the laser suffers internal reflections in the grid polarizers that leads to undesirable illumination returning towards the source.
As such there remains the need for a polarizer exhibiting improved transmitted illumination in combination with a substantially suppressed orthogonal-polarization reflection illumination.